The present invention relates to a method of monitoring the molding process of resin molding within a metal mold.
Molding of resin by using a metal mold is extremely common, including injection molding, transfer molding, compression molding, or the like. Such operations flow, fill and harden the resin in a cavity within the metal mold to form moldings. Particularly, in injection molding and transfer molding, the resin is caused to flow through a narrow area, which is called sprue, runner, gate or the like.
Roughly speaking, it is easy to repeatedly produce moldings of precise size, surface shape, strength or the like during such a resin molding operation. Thus, by filling constant quantities of material into a given shape of a metal mold cavity, and identical moldings always are produced. However, as the ranges of allowable tolerances of size, shape, strength and so on are reduced, it has become extremely difficult to repeatedly produce resin moldings with better precision.
This is why resin viscosity, repeating precision of the operation of the molding machine, the temperature of the metal mold have to be taken into consideration. However, the sequence of cause and effect of such factors is not explained in theory, but is explained through the accumulation of the respective actual examples.
Under such conditions, many proposals have been made of improving the accuracy of repetition in the production of moldings. Known detection systems chiefly are divided into two types. One system is to detect parameters of operating mechanisms, for example, screw speed, screw driving hydraulic pressure, screw revolution number, plunger speed and so on, while the other system is to detect parameters such as, for example, resin temperature, resin pressure, flow speed and so on, of the resin within the metal mold.
Action in response to such detected values also is divided into two types. One is to perform a feedback into the operation of the molding machine, while the other is to estimate the tolerance of the molding for a molding shot to determine quality.
The system of detecting parameters of the operation of the machine is easier to achieve feedback into the operation of the machine, but has a disadvantage that reproducibility is not strictly checked, because the action of the resin will be uncertain due to factors such as resin compressibility, friction with the machine, and so on.
The system of detecting parameters of the resin within the metal mold is advantageous because it is directly related to phenomena actually occurring within the mold.
A system of detecting the pressure within the metal mold is provided as one example of such system. The method described in Japanese Patent Publication (examined) No. 52-44346 detects the resin pressure near at the furthest portion within the metal mold from the inlet to control retention pressure. Also, in Japanese Laid-Open Patent Application (unexamined) No. 53-120769, resin pressures both near the sprue or gate and near the furthest portion from the inlet are detected, and a switch in pressure is detected when the pressure difference changes from decreasing to increasing during continuous measurement of such difference.
A system of detecting the speed of flow of resin flowing within a metal mold is another system. In Japanese Patent Publication (examined) No. 57-30658, two resin detectors are provided in a flow passage, and the speed of flow is detected by measurement of the time for flow between two points to perform a feedback operation into the molding conditions. In the method of Japanese Patent Publication (examined) No. 56-7862, several temperature sensing elements are disposed within the metal mold to detect the tip-end position, i.e., what is referred to as a melt front, of the resin flow to control the molding machine.
Though these mold interior detecting systems have the advantage of detecting actual operating conditions, they are slightly inferior with respect to repetition accuracy of the molding operation, so that the object of the detection operation cannot be sufficiently achieved. The correlation factor between the weight of a polyacetal molding of, for example, 2 mm in thickness, 45 mm in length and about 1 g in weight and the mold interior resin pressure was 0.07 in a normal molding machine. It is apparent that the effect is small if the pressure control of the molding machine is achieved with a pressure sensor provided within the mold in a case like this.
It has become obvious that the resin temperature and the resin viscosity greatly contribute towards the reproducibility of the moldings. Of course, as the background where such conclusion is drawn, the reliability of the mechanism of the molding machine is sufficiently increased at present, and the repetition accuracy of the control of the molding machine is improved. Also, it may be applied to a case where the repetition accuracy of the molding is expected to be further increased. It is found out that the correlation factor increases if the tolerance range of the mold interior pressure becomes larger.
A method of measuring the temperature and viscosity of the resin immediately before the filling operation into the metal mold located within the molding machine may be considered from the background described hereinabove. The method of Japanese Patent Publication (examined) No. 60-46008 measures the apparent viscosity of the resin from the screw action, for example, torque, to perform a feedback control of the molding conditions.
However, the viscosity of the resin is determined from the relation between the shearing speed and the temperature. The viscosity which is determined from the torque of the screw is not directly related to the viscosity of the resin flowing into the metal mold.
What is really required is the temperature, viscosity and shearing speed of the resin which starts to actually flow into the metal mold. These values are required to be detected a sufficient time before the resin is filled into the cavity of the mold.
Namely, in order to predict the tolerance range of the molding in the course of the molding process, the shearing speed which has an effect upon the viscosity of the resin flowing through a resin passage within the metal mold is obtained, and, furthermore, the resin temperature at such position is measured.